Back reflection typically occurs in a fiber optic system when light in an optical system enters or exits a fiber and when light enters or exits a fiber optic component. Typical fiber optic components include fused fiber optic components and micro-optic components.
Examples of fused fiber optic components are fiber optic taps and fiber optic wavelength division multiplexers. In fused fiber optic components the light stays confined to the optical fibers but at least one optical fiber is distorted by one of several processes usually involving heating the fiber while twisting it around another fiber.
Micro-optic components include a variety of very small optical devices in which a microlens substantially collimates the light and free space processing of the light can occur. For instance, in a wavelength division multiplexer, the collimated light interacts with a thin film filter that has been coated to reflect one color of light while transmitting another. In another example, a 1 by 2 switch can be made by directing the collimated light from a microlens into one of two output ports. Complex optical crossconnect switches can be made in a manner similar to the 1 by 2 switch, except that the number of input and output optical fibers can reach 1000 or more.
In the case of micro-optic components, back reflection occurs at every successive interface between regions with different refractive indices and in particular when the light leaves the optical fiber in transit towards the microlens element. More specifically, when light travels from glass to air or air to glass, a reflection occurs at the glass/air interface.
The application of an anti-reflection coating usually can reduce the reflections from about 4% (a typical reflection at an uncoated glass-to-air interface) to about 0.25% over the range of wavelengths used in most optical systems. Typical industry standards for back reflection into an optical fiber call for reflections less than 0.00032% (−55 dB). The intensity of any reflections needs to be very small because reflections back into the optical fiber can destabilize the lasers that are used to generate the light.
In the fiber optics industry, two methods are commonly used to reduce back reflections. One approach involves angle polishing the fiber at approximately eight degrees so that light reflected from the end of the fiber does not get reflected back into the fiber. U.S. Patent Publication 2002/0097956A1 by Kikuchi et al. describes the application of this approach to the interface between optical fibers and the substrate of a microlens array. The second method involves the use of an index-matching fluid or gel at the interface. Since these fluids are index-matched to the fiber, very little light is back reflected when joining two fibers of the same refractive index with a small amount of index-matching fluid or gel. The back reflections in these cases are often of the order of 0.0001% (−60 dB).
In the case of a complex optical crossconnect switch, the optical fibers providing input to the microlens array are typically held in place in a fiber block. Polishing the fiber block at 8-degrees would leave some fibers closer to the microlens array than others. The variable distances to the microlens array inhibits uniformly collimating all the beams of light.
In U.S. Pat. No. 6,212,316 to Presley, a standoff block with a matched refractive index is placed between the optical fiber and the bulk material into which the light is directed. Presley shows the optical fiber to terminate substantially perpendicular to the interface with the standoff block. Although this approach would avoid the problem of having the optical fibers terminate at varying distances from a microlens array, the use of a solid standoff block affixed to both the optical fiber and the bulk media makes fine adjustments during or after assembly difficult, if not impossible.
What is required is a simple device for reducing back reflections that overcomes some of the difficulties encountered with prior art devices.